Electrical resistor



May 17, 1949. T. HoLMl-:s 2,470,352

ELECTRICAL RES I STOR Filed July 20, 1945 JNVENToR. THOMAS HOL M55 ATTO/(W57.

Patented May 17, 1949 ELECTRICAL RESISTOR Thomas Holmes, London, England, assignor to The Hartford National Bank & Trust Company, Hartford, Conn., as trustee Application July 20, 1945, Serial No. 606,165 In Great Britain March 21, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires March 21, 1964 3 Claims. 1 This invention relates particularly to resistors produced by deposition of carbon from an atmosphere consisting of or containing a hydrocarbon gas or gas mixture, or a hydrocarbon vapour, or mixtures of these. n

Such resistors normally take the form of a ceramic or glass rod coated on the surface with carbon as aforesaid, provided with suitable end contacts, and nally sealed in an envelope or lacquered in such a way as to provide protection from atmospheric influences, such as humidity, etc.

Such resistors are not normally classed as insulated resistors unless provided with a suitable sleeving or moulded cover.

'Ihe present invention provides in a novel manner means whereby an insulated resistor may be manufactured without the need for the provision of a sleeving or moulded cover, so that the bulk is maintained as small as possible and the carbon coating is not exposed to the risk of damage during subsequent manufacturing stages.

According to the present invention the resistor is constituted by an insulating tube coated internally with carbon by deposition from a suitable atmosphere as defined above, the lead wires to the carbon coating being secured by means of electrically conducting cement which also serves to seal the ends of the tube.

The production of resistors and typical resistors according to the invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 shows a longitudinal section through a furnace Fig. 2 is a longitudinal section through a finished resistor Figs. 3 to 6 show alternative forms of lead wires.

For the production of the carbon coating on the inside of an open ended tube (which may be a ceramic, silica, glass or similar tube), the tube 2 is heated in a furnace 3 by means of a heating winding 4 to a temperature at which pyrolysis (of hydrocarbonslintroduced as described hereunder) will occur, and then passing along the tube an atmosphere containing a gaseous hydrocarbon (such as methane, ethane, propane, butane), or vapour of a hydrocarbon i such as benzene), or mixtures of these, with or without the admixture of a diluent such as nitrogen, via a tube l attached to one end of the tube 2 which is arranged to protrude from the furnace 3. Alternatively, the tube 2 may be iirst swept out with such an atmosphere, the flow of the gas being then maintained whilst the tube 2 is nextheated to the temperature at which cracking occurs. Variations of the above steps are obviously possible whereby the introduction of the gas atmosphere and the heating of the tube occur simultaneously, or the tube is first swept out with hydrocarbon or nitrogen or other inert gas or vapour, and the hydrocarbons introduced later, at such time, for example, as the tube is at the correct temperature for cracking or pyrolysis to occur.

After coating, the tube 2 is allowed to cool, either by switching ofi.' the heating winding 4 or removing the tube 2 from the furnace 3, the ow of hydrocarbon and/or diluent being maintained until the tube is cool, to avoid deterioration of the carbon film, as for example by oxidation.

A further alternative modification of the process consists in first evacuating the tube (sealed at one end), and heating it to the cracking temperature and then introducing the hydrocarbon or hydrocarbons with or without a diluent such as nitrogen, the tube later being cooled as above either with a vacuum, or hydrocarbon and/or diluent atmosphere. It is to be noted that this employment of a vacuum does not involve the use of a vacuum furnace, and is correspondingly easier to carry out.

A less satisfactory procedure is to coat the tube in a furnace in which the hydrocarbon entirely surrounds the tube both inside and out, as for example any furnace used for the production of the surface-coated rod type of pyrolytic carbon resistor. In this case it is then necessary to remove the carbon from the outside of the tube, after coating, in order to achieve the manufacture of a fully insulated resistor.

The process is best carried out using long lengths of the tubing to be coated, whereby that portion of the tube protruding from the furnace and which due to its relatively lower temperature is not coated, or is less thickly coated, with carbon, forms only a small portion of the whole length.

After coating and cooling, tubes coated as above may be divided into convenient shorter lengths for the further manufacturing steps.

These consist in the provision of end-contacts to the carbon film, and, if desired, the protection or insulation of the end-contacts, and lacquering to provide further protection and/or colouring of the resistor, e. g. for coding purposes.

Suitable end-contactsmay be provided by the insertion of eyelets or tags into the ends of the tube; to improve the contact, these eyelets or tags may be previously coated by the application of a carbon suspension such as known by the trade tube.

name of aquadag," or a copper-spray, or by silvering. The eyelets or tags are provided with wire leads.

Preferably, the internally coated tubes are provided with wire connecting leads B which are bent over at the ends (Figs. 2 and 3) or formed into a helix (Fig. 4) or similar shape (Figs. 5 and 6), the better to approach the inside diameter of the Before insertion into the ends oi the tubes, the wire ends, with or without eyelets, tags or shaping, are coated thickly with an electrically conducting cement t such as a paste of silvered copper or other suitable powder suspended in a solution of resin or similar binding material. The resin is preferably a thermosetting resin such as phenol-formaldehyde, cresol-formaldehyde, ureaiormaldehyde or oil-modifled poly-basic acid/- poly-hydric alcohol type or mixture of these, dissolved in a suitable solvent such as methylated spirit or solvent naptha.

The coated wire ends are now inserted into the ends oi' the resistor tube.

The resistors are allowed to air-dry for a. period of time, say 1 hour, at this stage, care being taken not to disturb the position of the contact wires. which are nevertheless substantially rmly held by virtue of the adhesive nature of the silveredcopper powder paste together with any resilience provided by the form of the end of the wire lead.

After drying, the resistors are baked, for eX- ample for i hour at 150 C., to cure the resin of the binder where a thermosetting resin or resins are employed.

Mechanically strong and electrically good joints are thus provided between the wire leads and the ends o! the resistance layer.

A further advantage is that an eiective seal is obtained at the ends of the resistor tube, protecting the conducting layer from atmospheric iniiuences.

If so desired, the ends of the resistors so produced may be covered by an insulating cement to protect from short circuits any residue of the conducting cement remaining on the end of the tube.

The resistor so produced may finally be coated with lacquer to further seal the ends and provide a means for coding the resistance values by colour.

I claim:

1. An electrical resistor comprising a hollow tubular member of insulating material, a coating of carbon on the internal wall of said tubular 4 member, connecting lead-in elements internally inserted into said tubular member, and a cement consisting of silvered copper powder suspended in a binder for securing said lead-in elements in position and simultaneously sealing the ends of said tubular member.

2. An electrical resistor comprising a hollow tubular member of insulating material, a coating of carbon on the internal wall of said tubular member, connecting lead-ln elements internally inserted into the ends of said tubular member, and

va cement consisting of silvered copper powder suspended in a thermcsettable resin for securing said lead-in elements in position and for simultaneously sealing the ends of said tubular menrber.

3. An electrical resistor comprising a hollow tubular member of insulating material, a coating of carbon on the internal wall of said tubular member, connecting lead-in elements internally inserted into said tubular member, and a cement consisting of silvered copper powder suspended in Vphenol-formaldehyde for securing said lead-in elements in position and simultaneously sealing the ends of said tubular member.

' THOMAS HOLMES.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,023,485 Thowless Apr. 16, 1912 1,453,724 Powell et al. May 1, 1923 1,711,974 Snelling May 7, 1929 1,751,587 Loewe Mar. 25, 1930 1,832,419 Pender Nov. 17, 1931 1,854,926 Broske Apr. 19, 1932l 1,965,059 Seibt I July 3, 1934 1,985,166 Haroldson Dec. 18, 1934 1,987,969 Parkin Jan. 15, 1935 1,998,060 Seibt II Apr. 16, 1935 2,027,277 Habann Jan. 7, 1936 2,273,056 McCarthy Feb.- 17, 1942 2,321,587 Davie et al June 15, 1943 2,338,531 Naumann et al. Jan. 4, 1944 FOREIGN PATENTS Number Country Date 820,167 France July 10, 1936 

